On Fri, Jun 18, 2021 at 1:57 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Minor comment nits:
>
> On Fri, Jun 18, 2021 at 04:48:14AM +0000, Jing Zhang wrote:
> > +/*
> > + * Common vm/vcpu stats read function to userspace.
>
> Should you use a real kernel-doc style here? You almost are, might as
> well do it "right" :)
>
Will fix that.
> > + * @id: identification string of the stats
> > + * @header: stats header for a vm or a vcpu
> > + * @desc: start address of an array of stats descriptors for a vm or a vcpu
> > + * @stats: start address of stats data block for a vm or a vcpu
> > + * @size_stats: the size of stats data block pointed by @stats
> > + * @user_buffer: start address of userspace buffer
> > + * @size: requested read size from userspace
> > + * @offset: the start position from which the content will be read for the
> > + * corresponding vm or vcp file descriptor
> > + *
> > + * The file content of a vm/vcpu file descriptor is now defined as below:
> > + * +-------------+
> > + * | Header |
> > + * +-------------+
> > + * | Descriptors |
> > + * +-------------+
> > + * | Stats Data |
> > + * +-------------+
>
> Where is the "header id string"? In the header?
>
Yes, the id string is in the header.
> > + * Although this function allows userspace to read any amount of data (as long
> > + * as in the limit) from any position, the typical usage would follow below
> > + * steps:
> > + * 1. Read header from offset 0. Get the offset of descriptors and stats data
> > + * and some other necessary information. This is a one-time work for the
> > + * lifecycle of the corresponding vm/vcpu stats fd.
> > + * 2. Read descriptors from its offset and discover all the stats by parsing
> > + * descriptors. This is a one-time work for the lifecycle of the
> > + * corresponding vm/vcpu stats fd.
> > + * 3. Periodically read stats data from its offset.
>
> You forgot "2.5. rewind fd pointer position", see below...
>
Sure, will clarify that.
> > + */
> > +ssize_t kvm_stats_read(char *id, struct kvm_stats_header *header,
> > + struct _kvm_stats_desc *desc, void *stats, size_t size_stats,
> > + char __user *user_buffer, size_t size, loff_t *offset)
> > +{
> > + ssize_t len;
> > + ssize_t copylen;
> > + ssize_t remain = size;
> > + size_t size_desc;
> > + size_t size_header;
> > + void *src;
> > + loff_t pos = *offset;
> > + char __user *dest = user_buffer;
> > +
> > + size_header = sizeof(*header);
> > + size_desc = header->count * sizeof(*desc);
> > +
> > + len = KVM_STATS_ID_MAXLEN + size_header + size_desc + size_stats - pos;
> > + len = min(len, remain);
> > + if (len <= 0)
> > + return 0;
> > + remain = len;
> > +
> > + /* Copy kvm stats header.
> > + * The header is the first block of content userspace usually read out.
> > + * The pos is 0 and the copylen and remain would be the size of header.
> > + * The copy of the header would be skipped if offset is larger than the
> > + * size of header. That usually happens when userspace reads stats
> > + * descriptors and stats data.
> > + */
>
> Looks like this is the networking "style" of multi-line comments, not
> the rest of the kernel. You might want to fix this up to be the normal
> style which would be:
>
> /*
> * Copy kvm stats header.
> * The header is the first block of content userspace usually read out.
> * The pos is 0 and the copylen and remain would be the size of header.
> * The copy of the header would be skipped if offset is larger than the
> * size of header. That usually happens when userspace reads stats
> * descriptors and stats data.
> */
>
> I do not know how picky the kvm maintainers are about this, that's up to
> them :)
>
>
Will fix it.
> > + copylen = size_header - pos;
> > + copylen = min(copylen, remain);
> > + if (copylen > 0) {
> > + src = (void *)header + pos;
> > + if (copy_to_user(dest, src, copylen))
> > + return -EFAULT;
> > + remain -= copylen;
> > + pos += copylen;
> > + dest += copylen;
> > + }
> > +
> > + /* Copy kvm stats header id string.
> > + * The id string is unique for every vm/vcpu, which is stored in kvm
> > + * and kvm_vcpu structure.
> > + */
>
> This header too is skipped if necessary, so you should say that as well.
>
>
Sure, will clarify that.
> > + copylen = size_header + KVM_STATS_ID_MAXLEN - pos;
> > + copylen = min(copylen, remain);
> > + if (copylen > 0) {
> > + src = id + pos - size_header;
> > + if (copy_to_user(dest, src, copylen))
> > + return -EFAULT;
> > + remain -= copylen;
> > + pos += copylen;
> > + dest += copylen;
> > + }
> > +
> > + /* Copy kvm stats descriptors.
> > + * The descriptors copy would be skipped in the typical case that
> > + * userspace periodically read stats data, since the pos would be
> > + * greater than the end address of descriptors
> > + * (header->header.desc_offset + size_desc) causing copylen <= 0.
> > + */
>
> But you say that it is skipped here.
>
> > + copylen = header->desc_offset + size_desc - pos;
> > + copylen = min(copylen, remain);
> > + if (copylen > 0) {
> > + src = (void *)desc + pos - header->desc_offset;
> > + if (copy_to_user(dest, src, copylen))
> > + return -EFAULT;
> > + remain -= copylen;
> > + pos += copylen;
> > + dest += copylen;
> > + }
> > +
> > + /* Copy kvm stats values */
> > + copylen = header->data_offset + size_stats - pos;
> > + copylen = min(copylen, remain);
> > + if (copylen > 0) {
> > + src = stats + pos - header->data_offset;
>
> This lets you sync to the end of the header and read just the stats, but
> does that mean that userspace keeps needing to "rewind" back to the end
> of the header to read the stats again?
>
> Or can it just keep reading off the end of the previous read?
>
> It's not quite obvious here, and I mention that above in step "2.5", so
> maybe I am wrong, which is fine, but then I'm confused :)
Userspace needs to rewind back to read the stats again or just use pread
as Paolo mentioned and that's used in the testcase.
>
>
> thanks,
>
> greg k-h
